I've been experiencing the same issue for several months now. I’m using a MacBook Pro 2017. I'm not sure if it's related to a recent OS update, the issue comes after updating new OS. I've tried resetting the SMC, PRAM, and NVRAM, but the issue persists. I consulted someone, and they suggested that my screen might be broken, but I don't think that’s the case—it seems like either a hardware or software issue.
The problem is that black horizontal lines only appear on the screen when it's static, meaning when I'm not using the keyboard or mouse. As soon as I interact with the keyboard or move the mouse, the black lines gradually disappear. If I connect an external monitor, the black horizontal lines also disappear completely.
I'm really confused and not sure what the problem is. Now I can't bring laptop to use anywhere since I have to connected it with external monitor to work.
Can anyone help with a solution?